1. Field
Embodiments of the invention generally relate to a method and apparatus for encapsulating an organic light emitting diode.
2. Description of the Related Art
Organic light emitting diode (OLED) displays have gained significant interest recently in display applications due to their faster response time, larger viewing angles, higher contrast, lighter weight, low power and amenability to flexible substrates such as compared to liquid crystal displays (LCD). However, OLED structures may have a limited lifetime, characterized by a decrease in electroluminescence efficiency and an increase in drive voltage. A main reason for the degradation of OLED structures is the formation of non-emissive dark spots due to moisture or oxygen ingress. For this reason, OLED structures are typically encapsulated by an organic layer sandwiched between inorganic layers. The organic layer is utilized to fill any voids or defects in the first inorganic layer such that the second inorganic layer has a substantially uniform surface or deposition.
FIGS. 1A-C illustrate a conventional process for depositing the encapsulating layers, typically including a first inorganic layer 106 (shown as 106a and 106b), an organic layer 108 (shown as 108a and 108b) and a second inorganic layer 116 (shown as 116a and 116b). The process begins by aligning a first mask 109 over a substrate 100 such that an OLED structure 104 is exposed through an opening 107 unprotected by the mask 109, as shown in FIG. 1A. The first mask 109 defines the opening 107 having a distance 110 from the OLED structure 104 to the edge of the first mask 109. The first mask 109 is typically made from a metal material, such as INVAR®. As illustrated in FIG. 1A, the first mask 109 is utilized to pattern the first inorganic layer 106 (shown as 106a, 106b), such as silicon nitride or aluminum oxide, over the OLED structure 104. The first mask 109 is positioned such that a portion 105 of a contact layer 102 adjacent to the OLED structure 104 is covered by the first mask 109 so that the inorganic layer 106 does not deposit on that portion 105. As illustrated in FIG. 1B, the first mask 109 is removed and replaced by a second mask 114 having an opening 111 smaller than the opening 107 of the first mask 109. The second mask 114 defines the opening 111 having a distance 112, which is shorter than the distance 110 as defined by the first mask 109, from the OLED structure 104 to the edge of the second mask 114. By utilizing the second mask 114, an organic layer 108 (shown as 108a, 108b) is deposited over the first inorganic layer 106. As the second mask 114 has the opening 111 smaller than the first mask 109, the organic layer 108 does not completely cover the underlying inorganic layer 106. The encapsulation of the OLED structure 104 is finished by depositing at least a second inorganic layer 116 (shown as 116a and 116b) over the top of the exposed portion of the first inorganic layer 106 and the organic layer 108 utilizing the first mask 109 as illustrated in FIG. 1C. The second inorganic layer 116 fully encapsulates the organic layer 108 with the first inorganic layer 106, thereby encapsulating the OLED structure 104 while leaving the portion 105 of the contact layer 102 exposed.
The conventional process flow described above has significant challenges preventing commercially viable scaling for use with larger area substrates, such as substrates having a top plan area greater than about 1,500 centimeters square. For example, the two metal masks 109, 114 required for implementing the above described process for such large area substrates are very expensive, and may each exceed $40,000.00 in cost. Additionally, very tight alignment tolerance of each metal mask 109, 114 to the OLED structure 104 is required, generally within 100 μm. As these masks 109, 114 often exceed one meter in length, the masks 109, 114 undergo significant thermal expansion when heated from ambient temperatures to processing temperatures of about 80 degrees Celsius. This significant thermal expansion provides a major challenge for OLED fabricators as to how to prevent alignment loss between the openings 107, 111 formed through the masks 109, 114 and the OLED structure 104. Loss of alignment may result in incomplete encapsulation of the OLED structure 104, which in turn leads to shortened life and diminished performance of the OLED device 104.
Therefore, an improved method and apparatus for thin film encapsulation of an OLED structure is needed.